1. Field of the Invention
The invention relates generally to building construction and more specifically to concrete footings and foundation wall systems for prefabricated manufactured buildings that are brought onto a construction site.
2. Description of the Prior Art
Manufactured buildings and homes are typically built in modular sections that are each permanently supported by a pair of "transporter" beams. The walls are not directly supported by such transporter beams, and yet represent one of the greatest loads. Sometimes gussets off of the transporter beams are used that reach out to points on the perimeter rim under the outside walls, but such measures are inadequate over the life of the building. A substantial portion of the total weight of a manufactured building is represented by the walls themselves and the roof loads carried by them.
Nevertheless, the manufacturers of such buildings invariably specify only the use of jacks and blocks that support the transporter beams, marriage ridge beam, etc., at the final construction site. A complete installation that satisfies the local, state, and federal building codes for manufactured buildings can and often is done without providing a solid foundation support directly beneath the outside perimeter walls. Such a wall could help to dramatically reduce the flexing stresses borne by the manufactured building, and add a large measure of protection against seismic damage and wind.
It has been customary for people who were buying manufactured houses to have a foundation contractor build a concrete foundation on-site according to plans supplied by the building manufacturer. In an ideal world this will work fine, but in the real world there are always variations in the manufacturing process that can lead to surprises when the building is finally delivered to the site. For example, a bay window can be included that will not occur in the standard plan but will have to be provided for in the actual installation of the foundation because the outside wall and flooring outline are changed. Most factories simply supply generic plans that do not reflect custom details.
In general, federal and state building codes require manufactured homes to be permanently attached to a foundation by anchoring devices that are adequate to resist all the loads, for example as identified in the Code of Federal Regulations, 24 CFR 200.926d. Such can be satisfied without a perimeter support, e.g., supporting just the transporter beams. These requirements specify the minimum resistance to ground movements, seismic shaking, potential shearing, overturning and uplift loads caused by wind, earthquake, etc. Anchoring straps or cables affixed to ground anchors other than footings or piers will generally not meet the requirements. The manufactured building unit must be anchored to the footing or pier, e.g., forty such piers each rated for 5,000 pounds are typical in an ordinary installation. Permanent utilities must be installed and protected from freezing. And in most cases, towing hitches, running gear including tongues, axles, brakes, wheels, and lights, must be removed, leaving behind the chassis which must stay in place.
In general, the law requires that the crawl spaces be enclosed with a continuous permanent foundation-type construction similar to a conventionally built foundation, e.g., concrete, masonry or treated wood. If the perimeter enclosure is separate from the supporting foundation, it must be designed to resist all the credible forces to which it may be subjected to. The manufactured unit must be secured to the perimeter of the unit to exclude the entry of vermin and water, and yet allow good ventilation of the crawl space. Any movements or effects caused by frost heave, soil settlement consolidation, or the shrinking or swelling of expansive soils must not be transmitted to the building superstructure. Such perimeter structure generally is not actually required by the law to provide support to the building unit. However local building codes may require specialized techniques to suit the existing soil conditions.